Particularly in industrial settings, valves are used to carry and control a variety of gases, liquids and slurries over a wide range of temperatures and pressures. Many types of valves have emerged to meet the broad range of industrial applications. Some typical types of valves include plug valves, ball valves, butterfly valves, gate valves, check valves, etc. Often, when valves are intended to carry reactive chemicals or otherwise harsh media, the valves are built from exotic metals and materials. An alternative design to carry reactive chemicals or harsh media is to fully line the interior of a valve with a protective coating formed from a chemically inert material. Many valves are hand operated, while other valves have actuators to operate a valve from a remote location or to operate valves that are too large for human users to operate.
Invariably, a valve requires one or more seals to control the media passing through the valve and prevent leakage. Leakage can be categorized as internal or external. Internal leakage refers to fluid flow around a seal and back into the flowpath. An example of internal leakage is a valve in the closed position that nevertheless permits some fluid flow through the valve. On the other hand, external leakage refers to leakage from inside the valve to the external environment. Because the very nature of valves is to control fluid flow, either type of leakage is naturally undesirable.
An example of a typical seal in a plug valve 10 is illustrated in FIG. 1. A sleeve 13 is used to seal between a plug 11 and a valve body 12. Typically, the plug 11 is slightly tapered such that when it is forced down into the body 12, the sleeve 13 is compressed between the plug 11 and the body 12. When a user or actuator turns the stem 16, the plug 11 rotates relative to the body 12 and the sleeve 13. If the plug port 17 is aligned with the flow passage 15, the valve is in the open position and media is permitted to flow through the valve. If the plug port 17 is out of alignment with the flow passage 15, the valve is in the closed position and media is prevented from flowing through the valve. To facilitate tight sealing, the body 12 includes a series of continuous protrusions known as sealing ribs 14a, 14b, which are designed to interface with the sleeve 13. This plug valve 10 illustrates two types of sealing ribs: 1) port ribs 14a that encircle the flow passage 15 for preventing internal leakage, 2) circumferential sealing ribs 14b that encircle the plug 11 for preventing external leakage. Plug valves usually include a top seal (not shown) to further prevent external leakage and a cover (not shown) to hold the valve together.
A common set of materials used in valve seals are fluoroplastics, such as polytetrafluoroethylene ("PTFE"), fluorinated ethylene propylene ("FEP"), and perfluoroalkoxy ("PFA"). These materials offer several desirable characteristics, such as excellent chemical resistance and a low coefficient of friction. The chemical resistance of fluoroplastics permits these seals to retain their sealing integrity in a variety of applications, sometimes involving very aggressive and harsh chemicals. The low coefficient of friction in fluoroplastics facilitates less work on the part of users or actuators to operate the valve. As shown in FIG. 1, seals are often placed between rotating parts, which requires a torque (a rotational force) on the part of the user or actuator to move the parts. The lower the coefficient of friction of the seal material, the lower the torque needed to operate the valve.
Fluoroplastics also have less desirable characteristics. One such less desirable characteristic is its low resiliency, or plastic qualities. Plastic qualities, or plasticity, refers to a state where permanent deformations in the material occur under relatively low loads. For example, lead is a highly plastic material because under relatively low loads it will permanently deform. On the other hand, resilient qualities, or resiliency, refers to an elastic state capable of returning to an original shape after being bent, stretched, or compressed. An example of a resilient material is natural rubber, because after being deformed, it returns to its original state. Resiliency, or the lack thereof in plasticity, can be measured by the modulus of resilience using the equation 1/2(.sigma..sup.2 /E), where .sigma. is stress and E is the modulus of elasticity. Generally speaking, the higher the modulus of resilience, the more resilient the material, and the lower the modulus of resilience, the more plastic the material. Under the typical loads that a valve seal experiences, fluoroplastic materials tend to experience plastic deformations.
Fluoroplastics also experience time-dependant deformations under constant loads, a characteristic that is often referred to as cold flow, creep, or viscoelastic deformation. When fluoroplastic materials are utilized as seals, the breaking torque can be substantially greater than the running torque, an effect that is believed to be caused in part by cold flow. Breaking torque refers to the initial torque needed to operate a valve, whereas running torque refers to the subsequent torque needed to operate a valve. This breaking torque effect, which usually occurs after the seal has been compressed and stationary for a period of time, can be aggravated as the seal load increases.
Another undesirable characteristic of fluoroplastic seals is the fact that these materials have a different rate of thermal expansion than the other materials used in valves. All materials exhibit some degree of thermal expansion, which refers to the effect that dimensions change as temperatures change. When the materials in a valve have different rates of thermal expansion, the sealing ability can be jeopardized. For example, if the media in a valve experiences a drop in temperature, the metal portions will change dimensions at a different rate than the fluoroplastic seal. Because fluoroplastics are not very resilient, such seals cannot compensate for the changed dimensions. As a result, the seal could leak if the valve experiences significant enough temperature fluctuations. The possible loss of sealing can be further aggravated in higher temperature thermal cycling because creep induced plastic flow is exaggerated at such temperatures.